As photosensitive lithographic printing plates, there have been widely employed PS plates comprising a hydrophilic support and a lipophilic photosensitive resin layer formed thereon. As a plate-making method therefor, it has been a common practice to perform mask-exposure (face exposure) via a lith film followed by the dissolution and removal of non-image parts, thereby giving a desired printing plate.
As photosensitive lithographic printing plate precursors, there have been widely employed PS plates comprising a hydrophilic support and a lipophilic photosensitive resin layer formed thereon. As a plate-making method therefor, it has been a common practice to perform mask-exposure (face exposure) via a lith film followed by the dissolution and removal of non-image parts, thereby giving a desired printing plate. In recent years, digitization techniques whereby image data are electronically processed, stored and output with the use of a computer have been widely diffused. Thus, various image-outputting systems for these digitization techniques have been put into practical use. As a result, it has been urgently required to establish computer-to-plate (CTP) technology whereby an image is scanned with highly directional light such as laser beams in accordance with digitized data and thus a printing plate is directly made without resort to a lith film. Accordingly, it is an important technical problem to provide a lithographic printing plate precursor appropriate for the technology.
As such lithographic printing plate precursors usable in scanning exposure, there has been proposed a construction comprising a hydrophilic support and a lipophilic photosensitive resin layer (hereinafter also called a photosensitive layer) containing a photosensitive compound, which can generate an active species such as a radical or bronsted acid upon laser-exposure, formed on the support and such a product has been already put on the market. When a lithographic printing plate precursor of this type is laser-scanned in accordance with digital data, an active species thus generated induces a physical or chemical change in the photosensitive layer and insolubilizes the same. Next, the lithographic printing plate precursor is subjected to development, thereby giving a lithographic printing plate of the negative type. Among all, a lithographic printing plate precursor having a hydrophilic support and a photosensitive layer of the photopolymerization type, which is formed on the support and contains a photopolymerization initiator showing a favorable photosensitization speed, an ethylenically unsaturated compound capable of undergoing addition polymerization and a binder polymer soluble in alkali developing solution, together with, if needed, an oxygen-blocking protective layer has advantages such as a high productivity, convenience in development, high resolution and favorable ink impression and, therefore, available as a printing plate having desirable printing performance.
To improve press life of printing plates, use has been made of urethane resin binders (see, for example, JP-B-7-120040, JP-B-7-120041, JP-B-7-120042, JP-B-8-12424, JP-A-63-287944, JP-A-63-287947 and JP-A-1-271741). In these documents, however, no description is made on image drawing by laser-scanning exposure. Although there are lithographic printing plates which contain a urethane binder and are suitable for laser-scanning exposure, these plates are still insufficient in press life energy per unit area is lowered and thus the discrimination of exposed part/unexposed part is lessened. As a result, there arises a problem that the image part is frequently damaged by an alkali component in the developing solution and thus a favorable press life cannot be obtained.